1. Field of the Invention
The present invention relates to a phosphor mixture used for an illumination device and the like such as a light-emitting diode (LED), a vacuum fluorescent display, a fluorescent lamp, and the like, and an light emitting device using the phosphor mixture.
2. Description of the Relevant Art
As a light emitting device which has been conventionally used for an illumination device, a discharge-type fluorescent lamp, an incandescent lamp and the like can be cited. However, a light-emitting diode emitting a white light has been recently developed. The white LED illumination is an ideal illumination device having such merits of lower heat generation, better luminous efficacy, and lower power consumption compared to a conventional light source for illumination, and a long life without burnt-out of a filament like a conventional incandescent lamp because it is formed of only a semiconductor or a semiconductor and phosphor, and not requiring harmful material such as mercury which are maleficent to the environment but are essential to a conventional fluorescent lamp.
As necessary factors in a light source for general-purpose illumination including the white LED described above, color rendering can be cited as well as brightness. The color rendering is a value indicating reproducibility of color depending on a light source, and JIS Z 8726 (1990) is generally used for an evaluation method of the color rendering. Consequently, explanation will be made from now on based on the evaluation method of JIS Z 8726.
According to JIS Z 8726, the color rendering of a light source is expressed numerically by a general color rendering index (Ra). This is an index to evaluate difference in color between a reference sample for evaluating color rendering illuminated by a sample light source in test and the reference sample illuminated by a reference light close to a natural light. If both are identical without any difference, the color rendering index is 100. Even when color temperatures of a light source, are identical, color is seen differently depending on a color rendering index, and if the color rendering index is low, the color looks dull and dark. If a light source has the more uniform light intensity over the entire visible light region, the light source can be said to have a good color rendering.
The white LED becoming popular as a general-purpose illumination is generally an LED light source which emits light appearing white when seen by human eyes by synthesizing blue emitting light and yellow emitting light by combining a blue light emitting LED and a yellow light emitting phosphor excited by receiving the blue light thus emitted. However, since this type is composed of blue emitting light and yellow emitting light, it presents a problem that green and red which are required for a light source for illumination is extremely weak. Especially, a color component for a long-wavelength side or red in the visible light region is insufficient, which results in emitting a slightly bluish white light though it is still in white zone so far. Accordingly, when this white LED is used for a general-purpose illumination, a red object appears to be very dull red, which means color rendering of this light source is very unfavorable. Furthermore, as it has no red light-emission component, it is impossible to obtain reddish white emitting-light like warm white at a correlated color temperature of 4000 K or less.
A white LED for obtaining a white emitting-light from light in RGB etc. by combing an LED emitting a near-ultraviolet or ultraviolet light and a phosphor emitting a red light (R), a phosphor emitting a green light (G), a phosphor emitting a blue light (B), and phosphors emitting other colors excited by receiving the above-described near-ultraviolet or ultraviolet emitting-light has been developed to improve the above-described color rendering of white LED. It is possible for the white LED which obtains a white light from lights of the respective RGB phosphors to obtain an arbitrary light including a white light by combination or combination ratio of RGB phosphors. Besides, since the wavelength of the obtained emitting light contains almost all regions in the visible light region, it is possible to enhance the color rendering.
Existent phosphors have been studied to utilize as a phosphor in RGB and others used for this usage. For a red phosphor, there are, for instance, Y2O2S:Eu, La2O2S:Eu, 3.5MgO.0.5MgF2.GeO2:Mn, (La, Mn, Sm)2O2S.Ga2O3:Eu, SrS:Eu, CaS:Eu. For a green phosphor, there are, for instance, ZnS:Cu,Al, ZnS:Cu, SrAl2O4:Eu, BAM:Eu,Mn, (Ba, Sr, Ca)2SiO4:Eu. For a yellow phosphor, there is, for instance, YAG:Ce, and for a blue phosphor, there are, for instance, BAM:Eu (BaMgAl10O17:Eu), ZnS:Ag, (Sr, Ca, Ba, Mg)10(PO4)6Cl2:Eu. Researches to obtain a light source or an illumination device including an LED producing light of white or other desired colors by combining phosphors such as these RGB or others with a light-emitting section such as an LED emitting light in near-ultraviolet/ultraviolet have been carried out.
However, in white LED illumination by combination of a near-ultraviolet/ultraviolet LED and a phosphor in RGB or others, there is a problem that a red phosphor among RGB phosphors is especially low in emission efficiency compared with phosphors in other colors. Accordingly, when manufacturing a white LED, mixing ratio of the red phosphor is required to be extremely high compared with phosphors in other colors among the mixing ratio of the phosphors, which results in reduction of mixing ratios of the phosphors in other colors, so that a white color in high luminance cannot be obtained. Especially, as for a white LED requiring a light having a low color temperature such as warm white in which a mixing ratio of a long wavelength side (red color side) component is high, a necessary amount of a red phosphor is further increased, which causes a more pronounced problem that emission with high luminance as a whole cannot be obtained Therefore, in recent years, in order to solve the problem that exciting efficiency on a long wavelength side of the red phosphor described above is low compared with phosphors in other colors, a phosphor containing nitrogen such as silicon nitride series or the like is reported as a red phosphor which has a favorable excitation band on the long wavelength side, and can obtain a fluorescence peak having a wide half band width. The present inventors also propose Japanese Patent Publication Laid-open No. 2004-145718 (refer to Patent Documents 1 to 4). Light emitting devices which are improved in color rendering by combining light emitted from a blue LED and an emission of a yellow phosphor represented by YAG or the like, and an emission of a red phosphor containing above-described nitrogen or by combining a near-ultraviolet/ultraviolet LED with blue and green phosphors and a red phosphor containing above-described nitrogen are proposed (refer to Patent Documents 5 and 6).
(Patent Document 1) Translated National Publication of Patent Application No. 2003-515655
(Patent Document 2) Japanese Patent Publication Laid-open No. 2003-277746
(Patent Document 3) Translated National Publication of Patent Application No. 2003-515655
(Patent Document 4) Japanese Patent Publication Laid-open No. 2003-124527
(Patent Document 5) Translated National Publication of Patent Application No. 2004-505470
(Patent Document 6) Patent Bulletin WO 2004/039915 A1
A white light having favorable color rendering of which color temperature exceeds 5000 K has been obtained by combining the above-described nitride series red phosphor with a blue LED and a yellow phosphor such as YAG or the like, or by combining the above-described nitride series red phosphor, a phosphor in RGB or others and a light-emitting section such as a near-ultraviolet/ultraviolet LED.
However, based on the studies by the present inventors, it becomes clear that a light having favorable color rendering cannot be obtained even by the above-described method in a region of color temperature at 4500 K or less, especially in a reddish warm white of about 3000 K in color temperature, it is found that it is impossible to obtain a light in favorable color rendering. Besides, it is found that even in a white light by combining a near-ultraviolet/ultraviolet LED with a phosphor in RGB and others, though a white light in a region with a low color temperature can be obtained by using a known red phosphor as a red emission component, a light in favorable color rendering cannot be obtained. This may be due to shortage of light in a long-wavelength side of 630 nm or more.
Then, the present inventors studied known red phosphors which emit light in a long-wavelength side of 630 nm or more. However, it was found that such red phosphors were low in emission efficiency when excited by the light of blue LED or ultraviolet LED and couldn't obtain a light having high color rendering.
It is thought that as a demand from a future market, not only in a white light in high color temperature, but also in a warm white light in low color temperature, a light source such as an LED producing various light including a white light excellent in color rendering is required. However, it was found that it was insufficient to improve the color rendering with a phosphor mixture including a red phosphor of a conventional technology.